It is necessary to understand the role of the final motor pathway, the motor units, to comprehend how the nervous system controls limb movement. It has recently been shown that (1) the stimulus train which causes maximum tension-time integral to be developed by a motor unit is always characterized by an initial doublet followed by a steady-state discharge; (2) similar firing patterns are observed during motoneuron bursts during treadmill locomotion in the mesencephalic cat; and (3) recruitment of motor units occurs during locomotion as the demand for muscle force increases. This proposed research is to record the discharges of single units (and pairs of units) functionally isolated in intact ventral root filaments during locomotion controlled by brainstem stimulation and the tension-generated by these units to pulse trains. This data will be analyzed to determine whether (1) motor units fire with patterns for efficient generation of tension output; (2) motoneurons are recruited according to the size principle hypothesis; and (3) the rank order of excitability of a unit is immutable during locomotion and during reflexes.